1. Field of the Invention
The present invention relates to methods and apparatus for facilitating the operation of a closed-loop fiber optic gyroscope. More particularly, this invention pertains to methods and apparatus for obtaining accurate rate information at loop initialization.
2. Description of the Prior Art
The Sagnac interferometer is an instrument for determining rotation by measurement of the non-reciprocal phase difference generated between a pair of counterpropagating light beams. This instrument generally comprises a light source such as a laser, an optical waveguide consisting of several mirrors or a plurality of turns of optical fiber, a beamsplitter/combiner, a detector and a signal processor.
In an interferometer, the waves coming out of the beamsplitter counterpropagate along a single optical path. The optical waveguide is "reciprocal"; that is, any distortion of the optical path affects the counterpropagating beams similarly although they do not necessarily experience such perturbation at the same time or in the same direction. Time-varying perturbations may be observed where the time interval is comparable to the propagation time of the light around the optical waveguide whereas "non-reciprocal" perturbations affect the counterpropagating beams differently and according to the direction of propagation. Such non-reciprocal perturbations are occasioned by physical effects that disrupt the symmetry of the optical medium in which the two waves propagate. Two of the non-reciprocal effects are quite well known. The Faraday, or collinear magneto-optic effect, occurs when a magnetic field creates a preferential spin orientation of the electrons in an optical material whereas the Sagnac, or inertial relativistic effect, occurs when rotation of the interferometer with respect to an inertial frame breaks the symmetry of propagation time. The latter effect is employed as the principle of operation of both the ring and the fiber optic gyroscopes.
It is known that the fringe or interference pattern formed by the counterpropagating beams of a gyro consists of two elements, a d.c. component and a component that is related (e.g. cosine function) to the source of the phase difference between the beams. This phase difference provides a measure of the non-reciprocal perturbation due, for example, to rotation. As a consequence of the shape of the fringe pattern, when small phase differences are to be measured (e.g. low rotation rates), the intensity of the combined beam is relatively insensitive to phase difference as such difference occurs close to the maximum of the phase fringe pattern. Further, mere intensity of the composite beam does not indicate the sense or direction of rotation.
The above-described characteristics that result from the shape of the fringe pattern are commonly addressed by the superposition of an artificially biased phase difference upon the counterpropagating beams. The biasing of the phase shift, also known as "non-reciprocal null-shift," enhances the sensitivity of the intensity measurement to phase differences. A maximum degree of sensitivity is achieved by shifting the operating point of the gyroscope to .+-..pi./2 (or odd multiples thereof). Furthermore, by alternating the bias between +.pi./2 and -.pi./2, two different operating points are observed. This enables the system to separate the effects of phase differences from those of d.c. intensity changes.
In addition to phase modulation, the processing of an interferometer output commonly employs "phase nulling" that introduces an additional phase shift through a negative feedback mechanism to compensate for that due to the non-reciprocal (Sagnac) effect. Commonly, the negative feedback generates a phase ramp whose slope is proportional to the rate of rotation to be measured. In actual practice, a ramp whose height varies between 0 and 2.pi. radians is employed as the nulling phase shift cannot be increased indefinitely due to voltage constraints.
Various solutions to the problems associated with the design of closed loop or feedback systems for measuring rotation rate by means of a fiber optic gyroscope are disclosed in a number of U.S. patents. Ser. No. 4,705,399 of Graindorge et al. discloses a digitally-based arrangement that employs a "stairstep" waveform in which the height of each step is equal to the measured phase difference while the width or period of each is the group delay time of the optical coil. On the average, the slope of the ramp is equivalent to the measured non-reciprocal phase difference per unit of time. The phase modulation may be directly added to the digital ramp through the synchronization offered by a digital signal processor to generate a combined signal that controls a phase modulator. U.S. Pat. No. 5,337,143 of John G. Mark and Daniel A. Tazartes entitled "Loop Controller For Multiplexed Triaxial Gyro" discloses an application specific integrated circuit ("ASIC") that functions as a loop controller for a triaxial gyro. The controller accepts the digitized outputs of three modulated gyros, measures the rotation associated with each, digitally processes the outputs and provides analog signals for driving the gyro phase modulators. Microprocessor control adds a degree of flexibility, enabling the use of various types of modulation (e.g. random, pseudo-random, orthogonal, deterministic) and enhanced computational power for updating system parameters. U.S. Pat. No. 5,684,589 of John G. Mark and Daniel A. Tazartes entitled "Loop Controller For Fiber Optic Gyroscope With Distributed Data Processing" discloses a loop controller architecture that includes distinct units for distributing the necessary data processing functions, allowing operations to occur in parallel to enable additional useful gyro functions within each loop transit time. The gyro architecture eliminates any need for the gyro processor to perform throughput-intensive test and branch operations.
Pending U.S. patent application Ser. No. 08/893,961 of John G Mark and Daniel Tazartes entitled "Rate Control Loop For Fiber Optic Gyroscope" teaches a closed loop gyro configured to reduce the impact of so-called deadbeat residual error that is recognized as a significant contributor to bias error in, for example, high-g operational environments. This is accomplished by employing a rate controller within the feedback path that includes a cascaded plurality of feedback integrators. Such arrangement results in the assumption of a higher-order relationship of residual gyro error to sensed gyro rate.
Each of the above-described arrangements is subject to turn-on errors that are difficult to detect. An interferometric fiber optic gyro operates on a modulo 2.pi. basis in which a phase shift of .phi.+2n.pi. is indistinguishable to a first order from one of .phi., n being an integer. This, of course, follows from the cosine relationship that describes the interferometric relationship between intensity I and phase shift .phi.. Such relationship is preserved upon conversion to an electrical output at a photodetector. For gyros with large Sagnac scale factors (ratio of .DELTA..phi., phase offset to .theta., gyro rate) that operate over a substantial range of angular rates such inherent ambiguity can be troublesome, particularly at system turn-on. For example, a 1000 m gyro with a Sagnac scale factor of 3.5 .mu.rad./degree/hr. exhibits a phase shift of .+-..pi. at angular rates of .+-.250 degrees/second. The rate tracking of the closed-loop gyro system must therefore operate to values of greater than .+-..pi. if operation beyond .+-.250 degrees/second is anticipated, a scenario that is present in the case of virtually all present-day high performance aircraft.
In the above-described closed-loop prior art gyro arrangements it is assumed that the closed-loop system will power up in all cases within the zeroeth mode or fringe defined by the range -.pi.&lt;.phi.&lt;.pi.. As the rate increases during operation of a craft, changes in phase are registered and accumulated within a feedback integrator so that values well outside the zeroeth fringe can be tracked within the feedback integrator of the loop. Each of the prior art arrangements includes apparatus such that the value fed back will properly remain within the zeroeth fringe so that errors and voltages for driving the phase modulator(s) are minimized. For example, in U.S. Pat. No. 5,684,589, the binary word representing the phase ramp integrator is scaled such that it will accommodate only the range -.pi. to .pi.. Values added or subtracted to this integrator may cause overflows that exactly represent rollovers in the amount of .+-.2.pi., thus preserving the modulo 2.pi. operation. A left shift operation between the primary feedback integrator and the phase ramp integrator also ensures that the range of the feedback integrator is substantially greater than .+-..pi..
Generally, the assumption of rate within the zeroeth fringe upon initialization works well. However, various sources can produce false fringe capture upon power-up. This can occur, for example, in the presence of large signals at initialization that can lead to an accumulation in the feedback integrator outside the zeroeth fringe. As the gyro is insensitive to excess numbers of .+-.2.pi. or multiples thereof, the tracking loop can thereafter be completely stable despite providing a large rate estimate (and, hence, error) due to the initial false phase accumulation within the feedback integrator.
The prior art has addressed the problem of accumulation-induced error by clearing the phase-tracking feedback integrator some time after initialization, then recapturing the loop once the signals have settled. This approach has the undesirable consequence of requiring loop re-capture that can consume valuable time. The loop of pending U.S. patent application Ser. No. 08/893,961, which, as mentioned above, discloses a gyro arrangement for higher order operation, unfortunately leads to even more difficult initial loop capture procedures as simply resetting the phase-tracking accumulator to zero will generate a transient that can cause the loop to drive to a large value before recapturing. This re-opens the possibility of operating on the wrong fringe.